Materials having a high dimensional and/or thermal stability are required in many applications. Though plastic films consisting of organic polymers can be used as support for carrying a functional layer such as an image recording layer, several applications require the use of glass as dimensionally stable support for carrying said functional layer. Known examples thereof are graphic arts applications and photomasks for the manufacture of printed circuit boards. Another advantage of glass compared to plastic supports is the ease of recycling. An example of an application where high thermal stability is needed is the manufacture of colour filters in liquid crystal displays (LCD's), wherein glass plates are used as support in spite of the higher specific weight of glass compared to plastic supports. Due to the low flexibility of glass, the coating with a functional layer is typically carried out in a batch process (sheet by sheet), whereas the coating of a plastic support is generally performed as a continuous process, e.g. using a web coater.
EP-A 716 339 describes a process for providing a flexible glass support with a functional layer in a continuous coating machine, characterised in that (i) the thickness of the glass support is lower than 1.2 mm and (ii) the glass support has a failure stress (under tensile stress) equal to or higher than 1.times.10.sup.7 Pa and an elasticity modulus (Young's modulus) equal to or lower than 1.times.10.sup.11 Pa. The glass disclosed in the latter EP-A is quite brittle. Such glass is therefore not well suited for being wound on a core (e.g. after manufacturing the thin glass) or to be unwound from said core (e.g. for providing the thin glass support with a functional layer in a web coater) because the probability of glass fracture is high.
Chemically strengthened float glass is known to have greater strength than regular float glass. Chemically strengthened glass is glass wherein at both surface layers the original alkali ions are at least partially replaced by alkali ions having a larger radius. Known methods for producing chemically strengthened glass are typically batch processes wherein glass sheets are exposed to ion exchange conditions as described in e.g. JP-A 56 041 859, GB 1 208 153 and U.S. Pat. No. 3,639,198. Batch processes are characterised by a low productivity and therefore not attractive for application on an industrial scale. As the chemical strengthening of glass sheets is typically carried out by immersing the sheets in a salt bath at high temperature, it is difficult to obtain a uniform degree of ion exchange over the whole surface of the sheet.
Typical glass plates used in flat panel displays such as LCD's have a thickness of about 1.1 mm. A glass substrate having a size of 360.times.465.times.1 mm wheighs about 500 g and two of such sheets are needed in one LCD. The screen yield is inversely dependent on the thickness of the glass substrate due to the above mentioned risk of glass fracture. As the screen size of flat panel displays as well as the market penetration of portable devices containing a flat panel display is rapidly increasing, there is a need for a thin, strong glass substrate, which may be provided with functional layers and electronic components used in such displays.